1. Field of the Invention
This invention relates to a method and a device for detecting and measuring the content of reducing gas, such as hydrogen sulfide, using a semiconductor type of sensors. Reducing gas depletes oxygen from the semiconductor sensors upon exposure thereto and lowers electrical resistance through the semiconductor component thereof.
2. Description of the Prior art
It is known to use semiconductor sensors for measuring gas, in particular for measuring hydrogen sulfide (H.sub.2 S) content existing in natural gas. U.S. Pat. Nos. 4,822,465 to Jones, et al.; 4,387,165 to Youngblood; 4,197,089 to Willis, et al.; 4,030,340 to Chang; 3,479,257 to Shaver; and the article entitled, "Electrochemical or Solid State H.sub.2 S Sensors: Which is Right for You?" by Cheryl L. Kaminski and Albert Poli, InTech June 1985, for instance, disclose sensors for detecting a particular type of gas such as H.sub.2 S. In addition, U.S. Pat. No. 4,590,789 discloses a device for calibrating a gas sensor.
Generally, the principle of operation of a semiconductor sensor is as follows:
Gases capable of depleting O.sub.2 from metal oxide semiconductor material is referred to as reducing gases. Metal oxide semiconductors absorb oxygen when exposed to air or gas containing oxygen and become oxidized at the surface thereof. The oxygen absorbed at the surface of the semiconductor generates an electrical field which repels the electrons from the surface.
A metal oxide semiconductor sensor is formed on a non-conducting substrate between two electrodes. Heating elements are also formed on the substrate to heat the semiconductor at a desired temperature. When the semiconductor is heated to certain temperature ranges, the reducing gas being monitored changes the conductivity of the metal oxide semiconductor. For example, upon exposure of the heated metal oxide semiconductor to reducing gas, such as H.sub.2 S, a measurable decrease in electrical resistance occurs through the metal oxide semiconductor. This change is believed to be caused by a combination of adsorption, oxidation/reduction, and anion exchange phenomena in the semiconductor material. Theoretically, under zero gas conditions (no exposure to gas being measured), O.sub.2 molecules which are adsorbed/oxidized to the surface of the semiconductor, tie up free electrons in the semiconductor, thereby inhibiting electrical flow (high resistance). Reducing gas such as H.sub.2 S replaces O.sub.2, releasing the free electrons and thus decreasing the resistance between the electrodes. The decrease in electrical resistance is related to the concentration of the gas component being detected and sampled. The surface of the metal oxide semiconductor behaves much like a "bio-mass", breathing in oxygen and oxidizing the surface thereof, and exhaling oxygen, for example, in the form of SO.sub.2 and H.sub.2 O in the presence of H.sub.2 S.
Over a period of time, O.sub.2 depletion will occur when the metal oxide semiconductor is exposed to gas such as H.sub.2 S. For proper operation, the presence of O.sub.2 is required and the sensor thus must be replenished with O.sub.2.
However, metal oxides semiconductors when exposed to air for prolonged periods without the presence of reducing gas such as H.sub.2 S tend to continuously oxidize and cause their electrical resistance to increase greatly. This phenomena is generally know as "going to sleep". This does not mean that the sensors will no longer be operative, rather their span of operation moves into a much higher electrical resistance range, most likely into the range beyond the measuring capability of their associated electronic hardware. If the metal oxide sensors are exposed to air or have been greatly oxidized, they need to be exposed to reducing gas for a relatively long period to remove some of the oxidation before reading from the sensor can take place, substantially increasing the response time for reading.
In order to replenish O.sub.2, the sensor must be exposed with ambient air or gas having oxygen for a predetermined time for the sensor to absorb O.sub.2 lost during sampling. During this oxidation period, in the prior systems, sampling of gas cannot be contemplated.
U.S. Pat. Nos. 3,924,442 to Kerho, et al.; 3,815,114 to Johnson, et al.; 3,678,513 to Ward, Jr.; and 3,039,053 and 2,965,842 both to Jacobson disclose a system for detecting gas using a plurality of sensors. However, none of these patents disclose the device and the method contemplated in the present invention.